The present invention relates to a metallized polyester film capacitor. Particularly, it relates to a metallized polyester film capacitor showing an enhanced adhesion between a base film and a deposited metal layer, and having an excellent moist heat resistance.
Polyester films, a typical example of which is polyethylene terephthalate film, are popularly used as base film for capacitors because of their excellent mechanical properties, heat resistance and electrical properties.
However, with the recent development of electronic devices, the polyester films have been required to have high performances. One of acute requirements in connection to such high performances is long-term stability to moist heat resistance. The metallized polyester films had the disadvantage of poor adhesion property between base film and deposited metal, especially under a high-temperature and high-humidity use environment. Therefore, the conventional metallized polyester film capacitors had the problem that although sheathed with an epoxy resin to a sufficient thickness, they would allow moisture penetrates at the interface between base film and deposited metal layer in the capacitor when they are left under a high-temperature and high-humidity condition for a long time, and this would cause corrosion of the metallized electrodes and large decline of capacitance. Thus, an improvement of moist heat resistance of the capacitors has been required for their long-term stabilized performance.
As an approach to such improvement, a method has been proposed in which instead of the coating with a low-viscosity epoxy resin under the atmospheric pressure, such coating-treatment is carried out under reduced pressure so that the treated resin would penetrate deep into the inside of the capacitor element by virtue of the difference in pressure vis-a-vis atmospheric pressure. This method, however, involves some serious problems. That is, the low-viscosity epoxy resin may be foamed under reduced pressure and may deposit on the lead wires, so that when it is tried to solder the capacitor devices to a printed wiring board, they may fail to be properly soldered on the board. Also, the above coating-treatment tends to weaken the self-healing of the film when a characteristic overvoltage of the plastic film capacitors is applied, and consequently the dielectric strength of the capacitor may be lowered.
The present inventors found that a polyester film having a coating layer composed of at least one of resins selected from polyester-based resins, polyurethane-based resins and polyacrylic-based resins, has a high adhesion properties of moist heat resistance to the deposited metal film, and that a capacitor device made by using such a metallized polyester film is possessed of high moist heat resistance in practical uses.
It is to be pointed out, however, that a polyester film coated with the said adhesive resin is excessively inferior in workability in the capacitor manufacturing process as compared with that of the conventional noncoated polyester films, specifically at the steps of depositing an electrode metal on the film, slitting of the film and winding-up of the metallized polyester film. It is to be further noted that with a recent tendency toward miniaturization of electronic devices, endeavors have been made for reducing thickness of the polyester films used as a dielectric for capacitors, and now improvement of film workability in the capacitor manufacturing process has become a matter of paramount importance.
As a result of the present inventors' studies to solve the above technical problems, it has been found that a film capacitor produced by using a metallized polyester film obtained by forming on at least one side of a polyester film a coating layer having a thickness (t) of 0.01 to 2 .mu.m and containing 0.1 to 20 wt % of the particles which satisfy the formula: 0.5.ltoreq.D/t.ltoreq.3 (D: average particle diameter), and depositing a metal on at least one side of the said coated polyester film, is excellent in adhesion between the polyester film as a base film and the deposited metal layer and high in moist heat resistance, and also has excellent electrical properties and good workability in the capacitor production. The present invention was attained on the basis of this finding.